1. Field of the Invention
This invention relates to a device for measuring the porosity of a filter element in the form of a substantially cylindrical-shaped rod or stick.
It applies notably, though not exclusively, to the measurement of draw through filter sticks used in filter-tipped cigarettes.
2. Description of the Prior Art
For this type of application, an encapsulation device has already been proposed using a rigid tubular element whose inner wall is lined with an elastic tubular membrane attached by its end edges to the two ends of said element so as to delimit an intermediate volume in which a partial vacuum can be created.
The cross section delimited by the membrane at rest is provided substantially equal to that of the filter to be measured, and slightly smaller than that of the tubular element.
In this way, the creation of a partial vacuum in the intermediate volume causes a retraction of the membrane which tends to apply itself against the inner wall of the tubular element. Accordingly, the cross section delimited by the membrane becomes greater than that of the filter which can thus fit unhindered into the assembly constituted by the two parts.
Once the filter has been disposed inside this assembly, the intermediate volume is returned to atmospheric pressure. The tubular membrane then spreads out like a sphincter, under then effects of its own elasticity, to apply itself against and "encapsulate" the filter.
The porosity or draw of the filter can then be determined by creating a partial vacuum at one of the orifices of the tubular element while the other one is left unconnected, and by taking e.g. a measurement of the pressure at a constant flow rate of the air sucked through the filter.
In order to enable filter elements of different lengths to be measured using a structure of this type, it has been proposed to adjust the useful length of the flexible membrane by means of a tubular sleeve susceptible of axial sliding inside the membrane so as to confine the expansion of the membrane over part of its length. The dimensions of this sleeve are provided so that, in this part, the diameter of the sphincter will be greater than that of the filter.
It so happens that a device of this type has a certain number of drawbacks.
Firstly, adjustment of the encapsulation length, which can be performed continuously, can only be performed in the control mode.
Moreover, it is difficult to automate the positioning of the tubular sleeve as a function of the length of the filter, as this would require a relatively complex tube displacement system and system for measurement thereof.
Furthermore, this device is necessarily cumbersome in terms of height since, in order to measure a filter encapsulated over its maximum length, the tube must be completely removed from the tubular membrane, thereby increasing the total height of the measurement head by said length.
It has also been observed that, in a same draw measurement cycle, it is not possible to successively measure the filter with total encapsulation and the filter with partial encapsulation in order to obtain a parameter depending on the porosity of the filter jacket.